This invention relates to combustion product detectors of the ionization chamber type, and more particularly to such detectors employing a single radioactive source.
Prior art combustion product detectors have been of two types; a static type having two separate ionization chambers with each chamber utilizing its own radioactive source, and a dynamic type having a single ionization chamber having a single radioactive source. In the two-chamber static device, one of the chambers, a reference chamber, must be sealed or air tight, and the other chamber, a sampling chamber, is open to ambient conditions and allows the introduction of products of combustion into its interior. The reference and sampling chambers are connected in an electrical alarm circuit, and the electrical circuitry signals an alarm condition when products of combustion exceeding a predetermined level enter the signal chamber and alter the voltage across the chamber signifying a dangerous smoke or fire condition. In this manner, the reference chamber provides an electrical reference by which the sampling chamber's voltage is compared to signal the alarm condition. The single-chamber dynamic device is connected to elaborate electrical circuitry for constantly modulating the electrical operating characteristics of the chamber and monitoring the changes in modulation as products of combustion enter the chamber. Upon a predetermined change, the alarm is provided to signal a dangerous condition. Single-chamber dynamic detectors secure reliable performance but unfortunately are very expensive as a result of the electrical circuitry necessary for its dynamic operation.
Conventional two-chamber static combustion product detectors provide highly reliable operation under most conditions. The electrical characteristics of the reference chamber can be precisely established or determined during manufacture thereby securing a high degree of alarm sensitivity when the sampling chamber receives combustion product. Certain disadvantages are apparent when the conventional two-chamber detectors are manufactured in large quantities. Two radioactive sources, one in the reference chamber and one in the sampling chamber, are required at an increased cost. The reference chamber must be precisely manufactured so that it is completely sealed or air tight to prevent the introduction of combustion product within that chamber, which usually involves much attention and cost. The reference chamber must be constructed of very high quality insulation to eliminate current leaks and material which may contaminate the radioactive source, either of which would cause an undesirable or unstable operating condition. Furthermore, either the energy of radioactive source or the reference chamber must be adjusted to provide optimum conditions to insure precise operation of the detector. However, once a conventional two-chamber static device is currently manufactured, it provides highly desirable precision alarm-signalling performance.
The problems associated with a two-chamber static detector may be avoided if a combustion product detector of the static-operation type having a single radioactive source within a single ionization chamber is employed. Such devices are known in the prior art but these combustion product detectors are not of the dual ionization type recognized in the prior art as reliable and precise in operation. One such single radioactive source ionization detector has a single ionization chamber in which a grid is located at a position which affords the greatest voltage change when combustion product is introduced into the chamber. This device operates on a space-charge-limiting-effect principle, and does not employ dual ionization characteristics. Such space-charge-limiting-effect devices have no provision for providing reference and sampling chambers or zones and therefore lacks the recognized desirability of prior art two-chamber static devices. Accordingly, it is an object of this invention to provide an improved ionization dual-zone static detector having a single radioactive source which provides reliable and precise performance.
It is another object of this invention to provide an improved ionization dual-zone static detector having only a single radioactive source.
It is another object of this invention to provide an improved ionization dual-zone static detector having a single radioactive source in which a single ionization chamber provides the characteristics of a reference zone and a sampling zone.
It is a further object of this invention to provide an improved ionization dual-zone static detector which does not require selection of a radioactive source or adjustment of the ionization chamber or internal components.
It is a further object of this invention to provide an improved ionization dual-zone static detector having a single radioactive source that does not require an air tight or sealed ionization chamber.
It is yet another object to provide an improved ionization dual-zone static detector having a single radioactive source which is significantly less expensive to manufacture than conventional two-chamber static detectors.
It is yet another object of this invention to provide an improved ionization dual-zone static detector having a single radioactive source which greatly reduces possible negative effects which may effect prior art detectors.
To achieve these and other objects the present invention employs a single radioactive source in an ionization chamber to provide dual-zone ionization. The ionization chamber includes a relatively small portion and a relatively large portion comprising reference and signal zones respectively. Means for applying voltage across and conducting current through the reference and signal zones are also provided within the ionization chamber. The single radioactive source provides an ion current which flows in both zones. The voltage applying and current conducting means is arranged within the ionization chamber to conduct maximum changes of voltage across the signal zone products of combustion modify the electrical characteristic of the signal zone. Provision is also made for connecting the reference and signal zones in series, for applying voltage across reference and signal zones in series, and for monitoring the voltage across the signal zone to provide an alarm indication whenever the density of products of combustion within the ionization chamber exceed a predetermined maximum.
The features of novelty which characterize this invention are recited with particularity in the annexed claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages will best be understood by reference to the following brief description of the drawings and detailed description of the invention.